The invention relates to a method for the manufacture of N,N'-bis-aroyl hydrazines by catalytic reaction of aryl carboxylic acid alkyl esters with hydrazine or aroyl hydrazine in an inert medium of low voldtility.
It is known to use derivatives of N,N'-bis-salicyloyl hydrazine to prevent thermo-oxidative copper-catalyzed degradation of polyolefins (U.S. Pat. Nos. 3,110,696; 3,843,492 and British Pat. No. 1,593,902).
According to known preparation methods (U.S. Pat. Nos. 3,849,492 and 3,993,622), N,N'-bis-salicyloyl hydrazines can be obtained by reaction of salycylic acid with hydrazide by adding thionyl chloride and pyridine to salicylic acid in chlorobenzene and then adding hydrazide. The products obtained still contain starting components and must be purified by recrystallization. The resulting N,N'-bis-salicyloyl hydrazine is employed as a metal deactivator for polymers. Its production in this manner, however, can cause eye irritation and eye damage because of exposure during preparation and use.
According to the method of U.S. application Ser. No. 173,409, N,N'-bis-salicyloyl hydrazines can also be produced by reaction of salicylic-acid alkyl esters with hydrazine or salicylic-acid hydrazide, optionally in the presence of nucleophilic and/or electrophilic catalysts. For N,N'-bis-salicyloyl hydrazine obtained in this way, eye irritations occur only in reduced form.
It is known from U.S. application Ser. No. 294,032 that the N,N'-bis-salicyloyl hydrazine yield can be increased considerably if hydrazine or salicylic acid hydrazide is reacted in one to ten fold excess relative to salicylic-acid alkyl ester and in the presence of a halide, hydroxide or oxide of boron, aluminum or zinc at temperatures up to 150.degree. C. If boron oxide is used as the catalyst, the product is obtained with especially high purity and, according to tests with rabbit eyes, irritates and damages the eye even less than the products obtained heretofore. The product N,N'-bis-salicyloyl hydrazine is present at the end of the reaction as crystal paste in excess alkyl ester. It is drawn off, washed with alcohol and dried in a vacuum.
With all these methods, N,N'-bis-salicyloyl hydrazine in powder form is eventually produced which can get into human eyes by dust formation during the work-up and technical processing, and can cause eye sickness.
It is, therefore, an object of the invention to prevent contamination of the personnel during the manufacture and technical utilization of N,N'-bis-salicyloyl hydrazines in such applications as metal deactivators for polymers. A further object is to obtain a simplification of the technical manufacturing method.